1. Field of the Invention
The present invention relates to a light weight small-sized thin battery to be mounted in portable electronic apparatus. The present invention relates to, e.g., lithium ion secondary battery which needs to be sealed so that it is free of water content, particularly a thin battery comprising a safety mechanism which can work when the battery body is simply sealed in the form of bag.
2. Description of the Related Art
FIG. 30 illustrates a schematic diagram of a prior art thin secondary battery disclosed in publications (e.g., "Kouseino Denchi no saishin manyuaru (Modern manual of high performance battery)", Sogo Gijutsu Center, page 185, 1989). In this drawing, the reference numeral 1 indicates a positive electrode, the reference numeral 2 indicates a negative electrode, the reference numeral 3 indicates a separator, the reference numeral 4 indicates a positive electrode collector tab, the reference numeral 5 indicates a negative electrode collector tab, the reference numeral 6 indicates an encapsulating bag, the reference numeral 7 indicates a bag sealing portion, the reference numeral 8 indicates a coating portion for metal bonding, the reference numeral 9 indicates a silicone oil, and the reference numeral 10 indicates a reinforcing resin plate.
In the foregoing example, the bag sealing portion 7 is bonded to the majority of the periphery of a small-sized sealed lead storage battery to seal a battery body 19 composed of the positive electrode 1, the negative electrode 2, the separator 3 and an electrolyte in the encapsulating bag 6 made of an insulating material for the purpose of reducing the size and weight of the case of the small-sized sealed lead storage battery. The collector tabs 4 and 5 are conductors for making electrical input/output between the internal electrode and the external circuit and are connected to the positive electrode 1 and the negative electrode 2, respectively. The coating for metal bonding 8 is provided to connect the collector tabs 4 and 5 to the encapsulating bag 6. Finally, the electrolyte is injected into the encapsulating bag 6 through the unsealed opening at the top of the battery body as viewed in FIG. 30. The opening is then sealed with the silicone oil 9 and the reinforcing resin plate 10. However, this battery is not equipped with a safety mechanism for blocking current passage upon the occurrence of troubles such as overcharging and shortcircuiting or relaxing the inner pressure of the encapsulating bag 6.
FIG. 31 is a sectional view illustrating a prior art hermetically-sealed can battery as disclosed in JP-A-6-140012 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). In the drawing, the reference numeral 11 indicates a positive electrode terminal plate, the reference numeral 12 indicates a negative electrode terminal plate, the reference numeral 14 indicates a safety valve, the reference numeral 15 indicates a spring, the reference numeral 16 indicates a gasket, the reference numeral 17 indicates an electrolyte, and the reference numeral 18 indicates a casing.
In the foregoing example, a cylindrical lithium ion battery comprising a positive electrode 1, a negative electrode 2, a separator 3 and the electrolyte 17 assembled into a battery body comprises a rubber elastic safety valve 14 supported by a spring 15 and provided interposed between the positive electrode terminal plate 11 and the sealing plate 13. The valve hole is normally closed. The pressure at which the valve begins to open is set to a predetermined value. In this arrangement, any accidents such as rupture due to the rise in the inner pressure of the battery can be prevented. Further, the deterioration of the battery body due to the opening of the battery can be inhibited.
FIG. 32 is a sectional view illustrating a prior art sealed can battery as disclosed in JP-A-6-96803. In this drawing, the reference numeral 21 indicates a positive electrode lead which electrically connects a positive electrode 1 to a sealing plate 13. The reference numeral 22 indicates an explosion-proof switch which normally is electrically connected to the sealing plate 13 at its point 22b. The reference numeral 23 indicates an insulating sheet. The reference numeral 24 indicates an insulating material.
In normal operation, the electrical connection between the positive electrode 1 and the positive electrode terminal plate 11 is secured by the positive electrode lead 21, the sealing plate 13 and the explosion-proof switch 22 to allow the battery body to act as a battery. When gas is produced from the interior of the battery due to overcharging or shortcircuiting, the inner pressure of the battery rises to cause the central part 22a of the explosion-proof switch 22 to deform. When the inner pressure of the battery reaches the predetermined value, the contact point 22b is released from electrical connection to the sealing plate 13. In this arrangement, current passage is blocked upon overcharging or shortcircuiting to prevent ignition or rupture of the battery.
In general, when gas is produced from the electrolyte or active material due to overcharging or shortcircuiting in a sealed battery encapsulating a battery body, the inner pressure of the sealed portion rises to possibly cause troubles such as ignition or rupture. Therefore, a safety valve for relaxing inner pressure as shown in FIG. 31 or a current blocking device as shown in FIG. 32 is needed. As in the conventional examples shown in FIGS. 31 and 32, various safety devices have been proposed for sealed can batteries comprising a casing prepared from a difficultly-deformable material such as metal and ABS.
In recent years, with the spread of portable electronic apparatus, further reduction of the weight, size and thickness of these portable electronic apparatus have been required. In order to simplify the casing or electrode terminal, a sealed battery comprising a battery body 19 simply sealed in the form of bag with an insulating coating material as shown in FIG. 30 has been developed. However, any small-sized light safety mechanism which makes the best use of this simple sealing has been little studied.
The secondary lithium battery which is under extensive development at present comprises lithium metal or lithium ion as a positive electrode active material. In this arrangement, a battery having a lighter weight which supplies a higher voltage than ever can be obtained. However, since such a positive electrode active material is weak to the invasion of water from the air, it is necessary that the entire battery body be completely sealed. Further, since lithium batteries have a high energy density, further care must be taken against accidents such as shortcircuiting.